1. Field of the Invention
The present invention relates generally to communications systems. More specifically, the present invention relates to a communication system in which an information-modulated electromagnetic wave has a carrier frequency and an electric field corresponding to a rotation vector tracing a nonlinear periodic path at a second frequency that is less than the carrier frequency of the wave.
2. Description of the Related Art
An electromagnetic wave can be defined by an electric field and a magnetic field that are orthogonal to one another along an axis of propagation. The behavior of the wave can be described with respect to the orientation of the field vector of the electric (E) field.
Polarization is a term that can be used to characterize the orientation of the field vector of an E field of some electromagnetic waves. Different types of polarization include: linear (also referred to as plane), circular, and elliptic polarization.
Where the field vector of an E field defines a plane as the wave propagates along an axis, the polarization of the wave can be referred to as linear or plane polarization. Where the terminus of the E field, i.e., the extremity of the field vector, traces a circular helix of period equal to the wavelength of the wave, the polarization can be referred to as circular polarization. Similarly, where the terminus of the E field forms an elliptical helix of period equal to the wavelength of the wave, the polarization can be referred to as elliptical polarization.
Polarized waves can be transmitted or received in a number of different ways. For example, an antenna itself can impose a certain polarization upon a transmitted wave or be sensitive to received waves of a certain polarization. A dipole antenna oriented horizontally with respect to the earth can receive and/or transmit linearly polarized waves where the plane of polarization is parallel to the earth. Similarly, a dipole antenna oriented vertically with respect to the earth can receive and/or transmit linearly polarized waves where the plane of polarization is perpendicular to the earth's surface. A helical antenna can receive and/or transmit circularly polarized waves.
Communications systems transmitting and receiving polarized waves can be adversely affected by apparent prolonged fading of transmitted/received waves having only one type of polarization. To minimize fading of the amplitude of the received wave having the one type of polarization, communication systems can simultaneously transmit and receive multiple waves each having a different polarization. This method can be characterized as polarization diversity.
Polarization has also been used to avoid inter-channel interference in geosynchronous satellite communications systems. A geosynchronous satellite can communicate with a ground station using right-hand circular polarized waves at a given carrier frequency, while an adjacent satellite can communicate with another ground station at the same carrier frequency using left-hand circular polarized wave. Helical antennas having opposite twists can be used to receive and/or transmit left-hand and right-hand circular polarized waves.
Polarization can be used to encode information in a communications system. U.S. Pat. No. 4,084,137, issued to Welti, describes a communications system that encodes a horizontally polarized wave and a vertically polarized wave in accordance with information. U.S. Statutory Invention Registration H484 describes a similar system that addresses a sidelobe problem in a radar system.
The polarization encoding concept addressed in the references described above can also be used to minimize the likelihood of unauthorized interception of a message. U.S. Pat. No. 5,592,177, issued to Barrett, describes a communications system that sequentially changes the polarization of a signal-carrying wave in a pseudorandom manner. The Barrett system provides broad polarization bandwidth for transmitting and/or receiving signals while minimizing the required frequency bandwidth of the transmitter and receiver systems. The selected polarizations include linear polarization with a variable polarization plane orientation, right-hand and left-hand circular polarizations, and right-hand and left-hand elliptical polarizations with a variable ellipse major axis orientation. By changing the specific polarization, the signal is spread in polarization in a manner analogous to the spreading of a signal over a continuous range of frequencies in spread spectrum communications systems. Note that when the signal-carrying wave is circularly or elliptically polarized, the field vector of the E field is rotating a frequency equal to the carrier frequency.
The concept of encoding a horizontally polarized wave and a vertically polarized wave can also be used to enhance channel discrimination in a two-channel communications system in which the channels have the same carrier frequency. U.S. Pat. No. 4,521,878, issued to Toyonaga, describes a communications system that encodes a horizontally polarized wave and a vertically polarized wave in accordance with a first code to form a signal corresponding to a first channel, and encodes a horizontally polarized wave and a vertically polarized wave in accordance with a second code to form a signal corresponding to a second channel. The system thus improves cross-polarization discrimination over known systems that attempt to simply transmit a first channel using horizontal polarization and a second channel using vertical polarization.
These known communications systems, however, suffer shortcomings. Regardless of the type of polarization used by the known communications systems, the E-field vector of an electromagnetic wave rotates at a rate (or angular velocity) that equals the carrier frequency of the wave.